1. Field of the Invention
The present invention relates to Traffic Alert and Collision Avoidance Systems (TCAS) and more particularly to detecting failures in the TCAS processor.
2. Description of the Prior Art
TCAS, the Traffic Alert and Collision Avoidance System, provides for safe separation of airborne traffic. To accomplish this, an airborne interrogator, the TCAS processor, interrogates an ATC transponder of other aircraft that are in the proximity of the TCAS equipped aircraft. The replies from the transponders of the other aircraft provide altitude and range of the other aircraft to the TCAS system. The TCAS equipped aircraft also carries an ATC transponder. In order to ensure that the transponder on the TCAS equipped aircraft does not reply to interrogations from its own TCAS it is equipped with a system called mutual suppression. The mutual suppression signal from the TCAS processor effectively disables the ATC transponder on the TCAS equipped aircraft at the time of interrogation. The ATC transponder is enabled again after the interrogation. The time that the transponder is suppressed is very brief (less than 90 microseconds).
The mutual suppression signal is applied to the transponder via a cable which connects the TCAS processor, the ATC transponder, and other avionics equipment which might be affected by the TCAS transmission. If this cable becomes disconnected, or the interface circuits in the transponder or TCAS processor fail to function, it is possible for the TCAS processor to receive replies from its own transponder. If the TCAS processor receives these replies consistently, it will interpret them as coming from a very close aircraft which is at the same altitude as its own aircraft. This will cause a spurious alarm to be presented to the pilot. It will seem as though there is another aircraft very close to the pilot's aircraft and he will be instructed to maneuver to avoid it. Since the other aircraft is actually his own, the other aircraft will seem to follow him no matter what action he takes.